In mammals, the spinal (or vertebral) column is one of the most important parts. The spinal column provides the main support necessary for mammals to stand, bend, and twist.
In humans, the spinal column is generally formed by individual interlocking vertebrae, which are classified into five segments, including (from head to tail) a cervical segment (vertebrae C1-C7), a thoracic segment (vertebrae T1-T12), a lumbar segment (vertebrae L1-L5), a sacrum segment (vertebrae S1-S5), and coccyx segment (vertebrate Co1-Co5). The cervical segment forms the neck, supports the head and neck, and allows for nodding, shaking and other movements of the head. The thoracic segment attaches to ribs to form the ribcage. The lumbar segment carries most of the weight of the upper body and provides a stable center of gravity during movement. The sacrum and coccyx make up the back walls of the pelvis.
Intervertebral discs are located between each of the movable vertebra. Each intervertebral disc typically includes a thick outer layer called the disc annulus, which includes a crisscrossing fibrous structure, and a disc nucleus, which is a soft gel-like structure located at the center of the disc. The intervertebral discs function to absorb force and allow for pivotal movement of adjacent vertebra with respect to each other.
In the vertebral column, the vertebrae increase in size as they progress from the cervical segment to the sacrum segment, becoming smaller in the coccyx. At maturity, the five sacral vertebrae typically fuse into one large bone, the sacrum, with no intervertebral discs. The last three to five coccygeal vertebrae (typically four) form the coccyx (or tailbone). Like the sacrum, the coccyx does not have any intervertebral discs.
Each vertebra is an irregular bone that varies in size according to its placement in the spinal column, spinal loading, posture and pathology. While the basic configuration of vertebrae varies, every vertebra has a body that consists of a large anterior middle portion called the centrum and a posterior vertebral arch called the neural arch. The upper and lower surfaces of the vertebra body give attachment to intervertebral discs. The posterior part of a vertebra forms a vertebral arch that typically consists of two pedicles, two laminae, and seven processes. The laminae give attachment to the ligament flava, and the pedicles have a shape that forms vertebral notches to form the intervertebral foramina when the vertebrae articulate. The foramina are the entry and exit passageways for spinal nerves. The body of the vertebra and the vertical arch form the vertebral foramen, which is a large, central opening that accommodates the spinal canal that encloses and protects the spinal cord.
The body of each vertebra is composed of cancellous bone that is covered by a thin coating of cortical bone. The cancellous bone is a spongy type of osseous tissue, and the cortical bone is a hard and dense type of osseous tissue. The vertebral arch and processes have thicker coverings of cortical bone.
The upper and lower surfaces of the vertebra body are flattened and rough. These surfaces are the vertebral endplates that are in direct contact with the intervertebral discs. The endplates are formed from a thickened layer of cancellous bone, with the top layer being denser. The endplates contain adjacent discs and evenly spread applied loads. The endplates also provide anchorage for the collagen fibers of the disc.
FIG. 1 shows a portion of a patient's spinal column 2, including vertebra 4 and intervertebral discs 6. As noted earlier, each disc 6 forms a fibrocartilaginous joint between adjacent vertebrae 4 so as to allow relative movement between adjacent vertebrae 4. Beyond enabling relative motion between adjacent vertebrae 4, each disc 6 acts as a shock absorber for the spinal column 2.
As noted earlier, each disc 6 comprises a fibrous exterior surrounding an inner gel-like center which cooperate to distribute pressure evenly across each disc 6, thereby preventing the development of stress concentrations that might otherwise damage and/or impair vertebrae 4 of spinal column 2. Discs 6 are, however, subject to various injuries and/or disorders which may interfere with a disc's ability to adequately distribute pressure and protect vertebrae 4. For example, disc herniation, degeneration, and infection of discs 6 may result in insufficient disc thickness and/or support to absorb and/or distribute forces imparted to spinal column 2. Disc degeneration, for example, may result when the inner gel-like center begins to dehydrate, which may result in a degenerated disc 8 having decreased thickness. This decreased thickness may limit the ability of degenerated disc 8 to absorb shock which, if left untreated, may result in pain and/or vertebral injury.
While pain medication, physical therapy, and other non-operative conditions may alleviate some symptoms, such interventions may not be sufficient for every patient. Accordingly, various procedures have been developed to surgically improve patient quality of life via abatement of pain and/or discomfort. Such procedures may include, discectomy and fusion procedures, such as, for example, anterior cervical interbody fusion (ACIF), anterior lumbar interbody fusion (ALIF), direct lateral interbody fusion (DLIF) (also known as XLIF), posterior lumbar interbody fusion (PLIF), and transforaminal lumbar interbody fusion (TLIF). During a discectomy, all or a portion of a damaged disc (for example, degenerated disc 8, shown in FIG. 1), is removed via an incision, typically under X-ray guidance.
Following the discectomy procedure, a medical professional may determine an appropriate size of an interbody device 9 (shown in FIG. 2) via one or more distractors and/or trials of various sizes. Each trial and/or distractor may be forcibly inserted between adjacent vertebrae 4. Upon determination of an appropriate size, one or more of an ACIF, ALIF, DLIF, PLIF, and/or TLIF may be performed by placing an appropriate interbody device 9 (such as, for example, a cage, a spacer, a block) between adjacent vertebrae 4 in the space formed by the removed degenerated disc 8. Placement of such interbody devices 9 within spinal column 2 may prevent spaces between adjacent vertebrae 4 from collapsing, thereby preventing adjacent vertebrae 4 from resting immediately on top of one another and inducing fracture of vertebra 4, impingement of the spinal cord, and/or pain. Additionally, such interbody devices 9 may facilitate fusion between adjacent vertebrae 4 by stabilizing adjacent vertebrae 4 relative to one another. Accordingly, as shown in FIG. 2, such interbody devices 9 often may include one or more bone screws 11 extending through interbody device 9 and into adjacent vertebrae 4.
Often, following the removal of the distractor and/or trial, a medical professional must prepare one or more bores or holes in a vertebra 4 intended to receive the bone screws 11. Such holes may be formed with the aid of a separate drill guide positioned proximate or abutting vertebra 4 and inserting a drill therethrough. Alternatively, such holes may be formed free hand, without the use of a drill guide. Further, since spinal column 2 is subject to dynamic forces, often changing with each slight movement of the patient, such screw(s) 11 have a tendency to back out (for example, unscrew) and/or dislodge from interbody device 9, thereby limiting interbody device's 9 ability to stabilize adjacent vertebrae 4, and consequently, promote fusion. Additionally, if screw(s) 11 back out and/or dislodge from the interbody device 9, they may inadvertently contact, damage, and/or irritate surrounding tissue. Further, interbody device 9 is commonly comprised of a radiopaque material so as to be visible in situ via x-ray and other similar imaging modalities. However, such materials may impede sagittal and/or coronal visibility, thereby preventing visual confirmation of placement and post-operative fusion.
Thus, there remains a need for improved interbody devices, associated systems, and methodologies related thereto.